1. Field of the Invention
The present invention relates to a semiconductor device such as a power-MOSFET or IGBT (insulated-gate type bipolar transistor) and, more particularly, to a semiconductor device that includes a protective element for protecting a gate insulation film from static electricity or another high voltage.
2. Description of the Related Art
Some semiconductor devices such as power-MOSFETs or IGBTs have a protective element 102 that uses Zener diodes inserted between a gate electrode G and source electrode S, as shown in FIG. 6, in order to prevent a gate insulation film from being destroyed by static electricity or another high voltage. In FIG. 6, D is a drain electrode. This protective element 102 is constituted having four stages in which inverted Zener diodes are alternately connected, for example, and prevents a gate insulation film from being destroyed by either a positive or a negative high voltage. The Zener diodes are not limited to a four-stage constitution. The constitution sometimes has two stages or six or more stages. The number of stages of a Zener diode and the doping concentration of the N-type (or P-type) impurity forming the Zener diode are the main cause of variations in the breakdown voltage and establish a breakdown voltage of between 20 and 30 volts, for example, which is a lower voltage than the upper limit voltage at which the gate insulation film withstands destruction (insulation withstand voltage).
Further, the gate insulation films of semiconductor devices of this type have grown thinner in accordance with the lower power supply voltages of semiconductor devices in recent years and therefore countermeasures to prevent destruction caused by static electricity or other high voltages have become even more important. Although such countermeasures may be considered to increase the surface area of the Zener diode of the above-mentioned protective element and to increase the electrostatic destruction resistance, these countermeasures cannot be said to be preferable because the countermeasures cause an increase in the chip size of the semiconductor device and, in turn, cause an increase in the costs of the semiconductor device.